Laser direct structuring processes (LDS) have been developed and used for the selective plating of molded plastic materials for more than 10 years, so called molded interconnect devices (MID). With LDS it is possible to realize highly functional circuit layouts on complex 3-dimensional substrates. The basis of the process involves additive doped thermoplastics or thermosets with inorganic fillers, which allow the formation of circuit traces by means of laser activation, followed by metallization using electroless plating. The metal containing additives incorporated in such plastics are activated by the laser beam and become active as a catalyst for electroless copper plating on the treated areas of the surface of plastics to be plated. In addition to activation, the laser treatment may create a microscopically rough surface to which the copper becomes firmly anchored during metallization. However, such technology is limited to apply on additive doped plastics, while general types of engineering plastic without additive doping cannot be activated for electroless copper plating.
Another technology in use is proprietary paint together with LDS. It is done by firstly spraying a thin layer of paint on the plastic parts. The LDS process then creates the circuitry layout on the paint coating and in the meantime activates the paint on the circuitry. The plastic will then go through electroless copper plating for metallization. This approach can be extended to plastics without additive doping. However, it is still in prototype stages and not yet ready for mass production.
Laser restructuring printing (LRP) is another innovative technology for the MID application. LRP employs high precision printing to create conductive diagrams (silver paste) on the workpiece to form the layout of the circuit. The printed workpiece is then laser trimmed. A high precision circuit structure is produced on the workpiece. This technology involves higher start-up investment on costly 3D printing machines.
Another technology is semi-additive process (SAP). The first step is to plate a thin layer of electroless copper on the plastic substrates employing existing colloidal catalyst and electroless copper for metallization on printed circuit board. A layer of negative electrodeposited photoresist is coated on the plastic substrates. Upon exposure and development, the circuit pattern is exposed without covering the photoresist. The exposed circuit can be plated with copper to achieve required thickness and then nickel. The remaining photoresist is removed. Excess copper layer is removed by micro-etch. An advantage of this technology is to be able to apply lower cost electrolytic plating processes for full copper build and nickel instead of the usual electroless plating processes. The plastic substrate is already fully plated with a layer of electroless copper. This technology can also be applied on plastic without doping additives. However, since it does not involve using lasers to roughen the circuitry, plating adhesion is a concern. In addition, the process sequence is quite long and complicated, with additional photoresist processes involved.
Although there are various processes relating to selective metallization of polymer and plastic materials, there is still a need for an improved method of selective metallization of polymers and plastics, in particular MIDs.